Research and development is under way for a simple high-speed/broadband communications technology.
A simple technique for wireless device-to-device communications is space communications involving, for example, radio waves (FRO communications) or light (optical space communications).
Many optical space communications techniques are usually implemented based on IrDA (Infrared Data Association) standards, ASK, and other schemes.
In these schemes, half-duplex communications, or a time-division based communications, are performed. Specifically, while one of two nodes is engaged in communications, the other node only receives a signal, transmitting no signal. When the communications stop, the nodes switch their transmission/reception roles.
However, every time the nodes switch their transmission/reception roles, half-duplex communications require a signal-absent period to free a receiver amplifier from overloading due to self-emitted light acting on the amplifier's optical reception capability. After that period, (i) a period for optical receiving components to become stable and (ii) a period to time output a preamble for synchronization of the CDR (Clock Data Recovery) for receiver circuitry to the signal must elapse before transmitting a start bit or synchronization signal. These wasted, non-communications periods reduce effective communications speed even at high data rates.
Accordingly, for efficient use of bandwidth, full duplex can be employed whereby both nodes can continuously engage in communications with each other.
However, optical full-duplex wireless communications have a problem that a node undesirably receives light transmitted by itself. This makes the technology extremely difficult in practice. A solution for light-based full-duplex wireless communications is given by conventional art. Details follow.
Patent documents 1 to 3 disclose use of differing wavelengths.
Patent documents 4 to 6 disclose electrical canceling of that part of outgoing signals which interferes with incoming signals.
Patent documents 7 to 10 disclose separating of optical axes by means of an optical system or block wall.
Patent documents 11 to 13 disclose application of properties of polarized light.
The following patent documents 1 to 16 show conventional art related to this application.
[Patent Document 1]
Japanese published unexamined patent application 64-64423 (Tokukaisho 64-64423/1989; published on Mar. 10, 1989)
[Patent Document 2]
Japanese published unexamined patent application 9-200152 (Tokukaihei 9-200152/1997; published on Jul. 31, 1997)
[Patent Document 3]
Japanese published unexamined patent application 11-196048 (Tokukaihei 11-196048/1999; published on Jul. 21, 1999)
[Patent Document 4]
Japanese published unexamined patent application 8-56198 (Tokukaihei 8-56198/1996; published on Feb. 27, 1996). An equivalent to Japanese patent 3137167.
[Patent Document 5]
Japanese published unexamined patent application 11-168435 (Tokukaihei 11-168435/1999; published on Jun. 22, 1999)
[Patent Document 6]
Japanese published unexamined patent application 8-56198 (Tokukaihei 8-56198/1996; published on Feb. 27, 1996)
[Patent Document 7]
Japanese published unexamined patent application 9-233026 (Tokukaihei 9-233026/1997; published on Sep. 5, 1997)
[Patent Document 8]
Japanese published unexamined patent application 11-234207 (Tokukaihei 11-234207/1999; published on Aug. 27, 1999)
[Patent Document 9]
Japanese published unexamined patent application 2000-244409 (Tokukai 2000-244409; published on Sep. 8, 2000)
[Patent Document 10]
Japanese published unexamined patent application 9-200134 (Tokukaihei 9-200134/1997; published on Jul. 31, 1997)
[Patent Document 11]
Japanese published unexamined patent application 11-239107 (Tokukaihei 11-239107/1999; published on Aug. 31, 1999)
[Patent Document 12]
Japanese published unexamined patent application 2000-357997 (Tokukai 2000-357997; published on Dec. 26, 2000)
[Patent Document 13]
Japanese published unexamined patent application 10-126343 (Tokukaihei 10-126343/1998; published on May 15, 1998)
[Patent Document 14]
Japanese published unexamined patent application 2001-292195 (Tokukai 2001-292195; published on Oct. 19, 2001)
[Patent Document 15]
Japanese published unexamined patent application 2001-308955 (Tokukai 2001-308955; published on Nov. 2, 2001)
[Patent Document 16]
Japanese published unexamined patent application 2002-84247 (Tokukai 2002-84247; published on Mar. 22, 2002)
[Patent Document 17]
Japanese published unexamined utility model application 4-35109 (Tokukai 4-35109/1992; published on Mar. 24, 1992)
However, conventional arrangements give rise to following problems.
In cases of differing wavelengths being used, two devices involved in communications may be designed to use respective wavelengths for transmission if they are paired up in advance. However, if there is no advance pairing, the devices need to be designed so that they are both capable of sending and receiving two different wavelengths. The need for such design adds to the cost of optical systems.
In cases of electrical canceling of unwanted interference of outgoing signals, signals from the transmitter circuitry are fed back to the receiver circuitry in an analog manner to cancel predicted interference signals from the transmitter circuit. The cancellation requires accurate prediction of return light which in turn necessitates provision of an optical system or an analog feedback circuit. These requirements add to the cost of the communications device.
In cases of separating of optical axes by means of an optical system or block wall, full duplex is achieved by, for example, narrowing beams with a lens so that the beams hit only the receiving end or separating outgoing light from incoming light. These processes require adjustment of the two axes or matching of angles and positions. Therefore, in these cases, either a separate optical system must be installed at an additional cost or the devices must be secured at a fixed angle and positions relative to each other.
In cases of application of properties of polarized light, polarizers are provided, one before the light source and another before the receiving section, so that the receiving section receives only the light from the other device through the polarizer. Light intensity drops when the light passes through the polarizer. In addition, the devices, to utilize polarized light, must be installed at a specific relative angle.